Question: A circle with circumference $20\pi$ has an arc with a $72^\circ$ central angle. What is the length of the arc? ${20\pi}$ ${72^\circ}$ $\color{#DF0030}{4\pi}$
Explanation: The ratio between the arc's central angle $\theta$ and $360^\circ$ is equal to the ratio between the arc length $s$ and the circle's circumference $c$ $\dfrac{\theta}{360^\circ} = \dfrac{s}{c}$ $\dfrac{72^\circ}{360^\circ} = \dfrac{s}{20\pi}$ $\dfrac{1}{5} = \dfrac{s}{20\pi}$ $\dfrac{1}{5} \times 20\pi = s$ $4\pi = s$